On the Name and operating system dialog set the Name field to a name you'd like to remeber this virtual machine by. Next set the Type to Linux and the Version to Ubuntu (64 bit) and then click Continue.

On the Memory size dialog, set the amount of your system's RAM (Random Access Memory) that you'd like to set the virtual machine to use.

On the Storage on physical hard drive dialog, accept the default setting of Dynamically allocated and click Continue.

On the File localtion and size dialog, make sure to set the disk space to an amount which will be suitable to your needs. If you'll be developing a lot of sites in this VM they may require more disk space, and you don't want to run out of disk space because resizing the partition later can be problematic. When you're finished, click Create.

Mounting the ISO Disk Image

Before you can install Ubuntu you need to download the ISO file for the version you'd like to install. Downloads are available at:

On the Storage tab, you can mount this ISO file under Controller: IDE by clicking the word "Empty" with the CD icon next to it. Then on the right side where it says CD/DVD Drive click the CD icon to the right and choose the Choose a virtual CD/DVD disk file... option. Lastly, accept these options to proceed and start the VM to run the ISO mounted as a CD in the drive so you can go through the Ubuntu install process.

Configuring Networking

Once you've installed Ubuntu, you'll need to configure networking so you can access your websites from your localhost. In Virtualbox, highlight your VM and click the Settings button. Navigate to the Network tab, and then click the Adapter 2 tab. Set Attached to: to the following:

You can now set up entries in your local machine's /etc/hosts file to create local hostnames by which to access the sites on your VM. Most people use website.localhost but you could use website.ubuntu or something else if you prefer. Add new lines like the following to your /etc/hosts to set this up:

website.localhost 192.168.56.101

Now just create an Apache VirtualHost by the same name and you can then visit http://website.localhost/ in your local machine's web browser.